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For systems consisting of molecules, the internal energy is partitioned among all of these types of motion. In systems consisting of monatomic particles, such as helium gas and other noble gases, the internal energy consists only of the kinetic energy of the individual atoms. Monatomic particles, of course, do not rotate or vibrate, and are not excited to higher electrical energies, except at very high temperatures.
Each molecular energy state has its own internal energy.
Internal energy can not be measured directly; it is only measured as a change (ΔU). The equation for change in internal energy is
where
Q is heat, measured in joules
W is work, measured in joules
When heat or work transfers energy from the system to the surroundings, Q or W is negative.
For quasistatic processes, the following relation holds:
where
T is the temperatureTemperature is the physical property of a system which underlies the common notions of "hot" and "cold"; the material with the higher temperature is said to be hotter. General description The formal properties of temperature are studied in thermodynamics., measured in kelvinThe kelvin (symbol: K is the SI unit of temperature, and is one of the seven SI base units. It is defined by two facts: zero kelvin is absolute zero (when molecular motion stops), and one kelvin is the fraction 1/273. 16 of the thermodynamic temperature o
S the entropyFor other uses of the term entropy see Entropy (disambiguation The thermodynamic entropy ''S often simply called the entropy in the context of chemistry and thermodynamics, is a measure of the amount of energy in a physical system which cannot be used to, measured in joule / kelvin
p the pressurePressure (symbol: p is a measure of force per unit area. where p is the pressure F is the force A is the area Often F is taken to be the of the magnitude of the mean vector force normal to the surface of area A upon which it exerts; the "surface" not nece, measured in pascals
μ the chemical potentialThe chemical potential of a thermodynamic system is the change in the energy of the sytem when an additional constituent particle is introduced, with the entropy and volume held fixed. If a system contains more than one species of particle, there is a sep, measured in
N the number of particles in the system, measured in
d denotes the differentialA differential can mean one of several things: Differential (mathematics) Differential (mechanics) Differential signaling is used to carry high speed digital signals. Differential (automotive)., the instantaneous change.